Phased array antennas or transducers are used for many purposes, including radars to detect and track targets, for sonar, for ultrasound and for sensing. A comprehensive description of phased arrays in radar and communication systems appear in a text entitled "Phased Array Antenna Handbook", authored by J. Mailloux, published by Artech House, Boston 1994, and incorporated herein by reference. Those skilled in the art know that antennas are reciprocal devices, and that characteristics of a particular antenna are same in both transmission and reception modes. Ordinarily a description of the operation of an antenna is couched in terms of either transmission or reception, with the other mode understood therefrom.
Those skilled in the art know that each array antenna produces undesired sidelobes in addition to one or more main lobes, and the sidelobes have fixed magnitudes if the antenna elements are uniformly illuminated. The magnitudes of the sidelobes can be controlled by control of the aperture illumination distribution (distribution of currents), but at the expense of loss of the directive gain or power of the antenna.
In addition to their military uses, array antennas are increasingly being used for commercial purposes, such as for airport terminal surveillance systems. An active antenna aperture is defined as one in which each receiving or transmitting element has its own amplifiers, power source, phase shifters and phase controllers, which are often combined into a transmit-receive (T/R or TR) module. The T/R modules of an array antenna may be expensive to manufacture, and the elements of the array may account for about one-third the cost of the radar. Commercial marketability is very price-dependent; any cost reduction is very desirable. Dramatic reductions in the cost and increases in the performance of a radar may be achieved by a) selectively thinning the array, thereby reducing the number of antenna elements and T/R modules, and locating the remaining elements for a given aperture in such a fashion as to reduce the sidelobe level without reducing the output power of each of the T/R modules and b) reducing ground or weather clutter by forming wide radar nulls in selected clutter regions without reducing the transmitted power (phase only control).